Mechanical filters are widely used in carrier transmission systems. When used in a channel translation stage, particularly in an FDM (frequency division multiplexing) system, the total number of mechanical filters used is very large, because such mechanical filters are used for every channel, and recent large capacity transmission systems may use up to 900 channels. Moreover, each mechanical filter includes several mechanical resonators which are its main component element, and the several mechanical resonators used in a filter are designed to have resonance frequencies which are incrementally different from each other.
The characteristics of a mechanical filter are notably dependent on whether the resonance frequency of each mechanical resonator matches its design frequency or not.
Therefore, it is necessary to adjust the resonance frequency of each mechanical resonator to its design frequency. Such adjustment is performed in two steps, namely coarse adjustment and fine adjustment.